1. Field of the Invention
The present invention relates generally to the fabrication of integrated circuits, and more particularly to a method for fabricating conductive structures in an integrated circuit.
2. Description of the Prior Art
Proper signal line and gate patterning is important to the operation of an integrated circuit. Reflections off the surface of a signal line or gate during exposure of an overlying photoresist layer, however, can subsequently cause problems with the formation of the signal line or gate.
Reflection problems are intensified on integrated circuits having a varied topography. The sidewalls of the hills and valleys cause the radiation utilized to expose a photoresist layer to reflect back into the photoresist at various angles, resulting in poor image resolution. The term radiation includes such photolithography methods as visible or ultraviolet light, x-ray, and electron beam. Poor image resolution is especially undesirable in complex integrated circuits because of the need to maintain critical dimensions. Furthermore, those skilled in the art will recognize that notching, or a narrowing of the signal line, typically occurs as a result of reflections. Notching is especially a problem when the patterned signal line crosses a step. As known in the art, notching can lead to electromigration in metals, premature punch through and short channel effects in transistors, and other device failure mechanisms.
One technique which is used to solve reflection problems is a dyed photoresist. This technique, however, is not completely successful in integrated circuits having a varied topography. As known in the art, the photoresist layer is thicker in the valleys than at other locations on the integrated circuit. Therefore, it is necessary to overexpose the photoresist in order to allow the radiation to reach the photoresist located at the bottom of the valleys. This overexposure, however, can increase the reflections from the signal line or gate into the photoresist layer, thereby increasing the problems of poor image resolution and notching.
Other techniques which are used to solve reflection problems include the use of an anti-reflective coating on the surface of the signal line, or on the bottom or the top of the photoresist layer. One example of an anti-reflective coating is a thin layer of oxide. A problem arises however, when it is necessary to electrically probe the integrated circuit as part of an in line electrical test. As known in the art, oxide is an insulating material. Capping conductive material with an insulating material makes it difficult to electrically probe the integrated circuit, thereby making it difficult to test the integrated circuit during fabrication.
Therefore, it would be desirable to provide a method for fabricating conductive structures in integrated circuits which reduces the reflections into a photoresist layer while maintaining a conductive upper surface on the conductive structures. It is also desirable that such a method not significantly increase the complexity of the fabrication process.